This problem is addressed with the setting up of data exchange between the power supply unit and light source module. Data exchange here means that the light source module transmits some information to the power supply unit relating to the current required by the module to meet its optical specifications or operating temperature for the purpose of reducing the value of the supplied current on exceeding a certain temperature limit value. Various ways are known for exchanging this information between the light source module and the power supply unit.
Buses can be used for exchanging data. Known, for example, are analog buses such as the 1 . . . 10 V interface or digital buses such as DALI (Digital Addressable Lighting Interface). Other known methods are simple resistor networks, which can be measured by the power supply unit and send to it the current requirement of the presently connected light source module or the presently connected light source modules.
DE 100 51 528 A1 discloses an interface of this kind in which a special resistor, known as a current setting resistor, is connected between a third line and the negative supply line. If several light source modules are connected to a single power supply unit, the resistors are connected to each other in series or in parallel, and in this way a sum signal is returned to the power supply unit in order to define the total current requirement. German patent application 10 2011 087 658.8 also discloses resistors for defining the current requirement of each individual light source module, i.e. module-specific current setting resistors.
The bus solutions have the drawback of two additionally required connection lines. The resistor solutions only require one additional connection line, but the evaluation of the resistor network and the resulting setting of the current value can become very complicated.
Since complete lighting systems including a power supply unit and light source module(s) have appeared on the market, many companies are attempting to follow a common path for bringing about communication between the two components of the above systems; equally for more complex high-end systems some digital protocols are being used, though the latter technology is not the background of the present invention and must be treated separately.
For example, the company Osram has already proposed an interface which is also able to provide support to an active circuit for thermal derating on a light source module. In this type of interface a current setting resistor on the light source module in connection with a pull-up resistor in the power supply unit forms a voltage divider with the aim of producing a midpoint voltage, which defines the output current of the power supply unit. An operational amplifier on the light source module starts to limit this midpoint voltage, and thereby the supplied operating current, as soon as the module overheats.
The company Philips has proposed another interface in which one signal line is connected to the current setting resistor and another signal line is connected to a temperature-sensitive resistor, and in which thermal derating is carried out by the power unit itself without requiring any active component on the light source module.
Both the aforementioned interfaces require a third additional line for the common signal mass feedback and use a voltage produced by the current setting resistor on the light source module for setting the nominal operating current value in that the operating current is set higher with increasing voltage over the current setting resistor or current setting resistors.
Recently the company Osram presented a slightly modified interface which is based on the aforementioned 1 . . . 10V bus but is modified by way of a precision power source in the power supply unit which makes it possible to achieve a precise nominal operating current value with just one simple current setting resistor per light source module. A further modification of this interface consists in replacing the current setting resistors on the light source modules with Zener diodes.
A new challenge is currently crystallizing out on the market, namely the possibility of connecting in parallel various modules and their common supply through one and the same power supply unit. The operating current supplied by this power supply unit must correspond to the sum of the nominal current values of all light source modules currently connected thereto, and the thermal derating capability must also be retained in multi-module arrangements. A thermal derating signal on a data line should ultimately even be dominant over a total current setting signal.
Nonetheless it is necessary to make the lighting systems simpler, which at present leads to a reduction in the number of additional data lines. Bus-based interfaces require at least four lines, two for the light source module operating current and at least two for the bus.
New properties for fulfilling the tasks are being considered:                Several modules should be able to be connected in parallel and supplied by one and the same power supply unit using the same interface. Through this the individual modules or at least those with identical operating voltages are considered as identical.        The interface for setting the operating current should have a reduced number of lines and for reasons of cost should be as simple as possible, in particular on the light source modules side.        
All previously presented and known interfaces are not able to correctly support multiple connections of light source modules. The evaluation circuits for the interface are also costly to produce.
In connection with this, the applicant has, for example in the unpublished document DE 10 2012 224 348.8, application date 21.12.2012, proposed a power unit and a light source module which have a simple interface for setting the current which is to be applied to the light source module. As said document has not yet been published, for the derivation of the object of the present invention, reference will initially be made below to the principles cited in said patent application. FIGS. 1 to 7 contained in the appendix and the associated description originate from said patent application. For the sake of clarity the corresponding digital embodiments will only be included in the disclosure content of the present application through reference.
The concept behind the invention set out in DE 10 2012 224 348.8 is always a three-line interface or an “analog one-wire interface” to which a light source module or several light source modules can be connected in parallel and connected to a single power supply unit, and the current requirements of each light source module are met in real time. The presented circuit configurations use a setting resistor in order to define a current value. For measuring this setting resistor various examples of forms of embodiment are described.
FIG. 1 shows the general concept of the setting resistors for the nominal operating current values. Three light source modules LEM are shown which are connected to a single power supply unit PSU. The connection consists of three lines: a supply line LED+, a common ground line LED− and a communications line CL. Each light source module LEM contains at least one LED chain. The LED chain includes a plurality of LEDs. According to the invention a plurality means that at least two LEDs are connected in series. Each light source module or each LED chain contains an individually allocated setting resistor for defining the nominal operating current valid in each case, known as the current setting resistor Rsetx. The current setting resistor Rset1 connects, or the current setting resistors Rset1, Rset2, Rsetm connect the common ground line LED− to the communications line CL outside a power supply unit PSU. This results in the parallel connection of all current setting resistors Rset1, Rset2, Rsetm present in the system so that the power supply unit PSU measures the equivalence resistor Rset of this parallel circuit. The concept means that the power supply unit PSU does not read a voltage as in the prior art, but a current that represents the conductance of this equivalence resistor. An inverse law is then applied to the value of the equivalence resistor in order to specify the value of the LED operating current to be supplied by the power supply unit. The law is as follows:
  Iout  =            Kv      Rset        .  
Kv has the dimension of a voltage. Rset is the value formed by a current setting resistor Rset1 or through the parallel connection of several current setting resistors Rset1, Rset2, Rsetm. As a result the value of the operating current supplied by the power supply unit is inversely proportional to the current setting resistor Rset1 or equivalence resistor Rset of the at least one light source module, i.e. the lower the ohm value of the equivalence resistor, the higher the output current of the power supply unit PSU. The requirement for the value of the operating current to ultimately correspond to the total of the nominal current values of each individual light source module is fulfilled by the known Ohm's law per se.
FIG. 2 shows a conceptual circuit diagram of an interface with thermal derating capability. Very simple thermal derating is achieved through placing a PTC element in series with Rset.
As soon as the temperature of the light source module LEM increases, the resistance value of the PTC increases and leads to a lower nominal current value for this module. The disadvantage of such an arrangement is that it will not be suitable for multiple connection of light source modules as the effect of a heated, solitary PTC would only remove the contribution of its assigned heated module from the conductance of the parallel-connected current setting resistors Rset, which is not sufficient for effectively reducing the temperature of the affected light source module. The colder current setting resistors connected in parallel counteract the temperature-related increase in resistance of an individual current setting resistor. The dominant nature of thermal derating is therefore not guaranteed.
Nevertheless, such a solution could be used for very inexpensive applications if a partial current reduction in the case of a temperature increase is still acceptable, for example in the case of light supply modules being supplied by at least one power supply unit or good thermal coupling between the light source modules. In addition, a simple thermosensitive element in series with the current setting resistor has the disadvantage of reducing its conductance and thus the value of the light source module current continuously, quasi linearly or gradually without defining a precise initiation point for thermal derating, even if certain PTC elements exhibit a very steep behavior around their nominal trigger temperature. The “nominal” current setting would thus be corrupted by a “parasitic” effect of the derating element.
FIG. 3 shows the concept of the three-line interface with a thermal derating unit TDU on the light source module. This concept is based on a different approach, namely providing a current source for the thermal derating unit TDU on the light source module. This current source is temperature controlled by way of a suitably connected thermosensitive element and, in order to avoid additional lines for the interface, is supplied with the required auxiliary power either directly by the supply line LED+ or from a center tap from the at least one LED chain of the light source module in question. The current source includes an amplifier and a temperature-sensitive resistor through which flows an input current for the amplifier which amplifies this input current to current ITDU of the current source. This current source has a response threshold which prevents any generation of a current ITDU until a particular excess temperature of the light source module is reached. Through this an increase in the amplified current with temperature (gradient of ITDU) is steep enough to successfully restrict the maximum temperature of a single overheated light source module in an entire system consisting of a power supply unit and several thermally independent light source modules without triggering instabilities due to heat transfer time displacements.
The current source for the current ITDU is capable of completely deactivating the signal formed by the equivalence resistor Rset of all parallel-connected current setting resistors: in this way it can reliably protect the entire system and, in particular, the light source module on which it is integrated, even in the case of a multiple connection of light source modules with simultaneous greatly concentrated overheating.
With the above-described temperature-dependent current source a further problem arises. It is necessary to measure the resistor Rset of module no. x independently of the actual temperature of module x, hence independently of the current supplied by the current source. It must be determined how the resistor Rset is to be measured in order to make the effect of the current source predictable. In the circuit configuration according to the invention a fixed voltage source Vk is used in order to measure the resistance value in that the circuit configuration applies the voltage of the voltage source via the current setting resistor Rset (or the parallel connection of several current setting resistors Rset) and reads the current flow brought about thereby. The voltage of the voltage source is thus emitted on the terminal for the communications line CL on power supply unit side. This in turn brings the thermal derating unit TDU into direct interaction with the current defined by Vk/Rset and resolves the ultimately set task of dominant thermal derating.
FIG. 4A shows a first variant of the light source module that provides the interface with just one bipolar transistor, an NTC element and some added resistors.
The circuit contains a voltage source V1 which is derived from the supply line LED+ of the light source module.
LEDs have a fairly stable forward voltage so that they can be used as an adequate voltage source replacement. In dependence on the supply voltage required for the thermal derating unit TDU the voltage source V1, always in relation to the common ground line LED−, can be connected to a tap between two sections of the plurality of series-connected LEDs. This means that the voltage V1 can be set in a way corresponding to a multiple of the forward voltage of an individual LED. In parallel to this voltage V1 there is a series connection of the NTC and a threshold resistor Rthr. The base of an NPN bipolar transistor (BJT) Q1 is connected to the node between the NTC and a resistor Rthr. The collector of Q1 is connected to the voltage V1. The emitter of Q1 coupled to the communications line CL via an emitter resistor Rtg. All the components of FIG. 4A described so far form the thermal derating unit TDU. The at least one current setting resistor Rset is connected between the communications line CL and the common ground line LED−.
In this circuit the emitter potential of Q1 is increased to a voltage (here Vk) determined by the power supply unit PSU, through which the threshold is reached, below which no current ITDU is injected into the communication line CL. If the temperature increases, the NTC starts to raise the base potential of Q1 until the NPN transistor Q1 reaches the active range.
As of now the emitter resistor Rtg defines the gain of the thermal derating unit TDU and thus the increase in injected current ITDU via the increase in temperature.
In relation to the voltages V1 and Vk the resistor Rthr and the resistance value of the NTC at the temperature specified as the trigger threshold for the TDU determine the initiation point for thermal derating. A further advantage of this arrangement is the good achievable linearity of the current ITDU over the temperature.
One of the most interesting advantages of this circuit configuration, in addition to the simplicity of its implementation on the part of the light source module, is, through setting the desired accuracies and features solely through the corresponding circuit complexity of the interface on the power supply unit side, that it is suitable for use in systems of different quality levels. In other words, it is possible to expand the read interface on the power supply unit side in accordance with the required precision and/or further necessary features.
FIG. 4B shows, as a second form of embodiment of the interface on the light source module LEM side, a complementary implementation. Here, a PNP bipolar transistor Q2 is used together with a PTC. A PTC is a temperature-sensitive resistor with a positive temperature coefficient. As in FIG. 4A, the voltage V1 is either derived from the total number of series-connected LEDs or from a part thereof. In contrast to the variant shown in FIG. 4A, the collector of Q2 forms the current source connection with the current ITCU which is connected to the CL. In this way the thermal derating threshold is no longer dependent on Vk, but now only on the easily reproducible voltage V1 as well as the values of the voltage divider formed by the temperature-sensitive resistance value of the PTC and the threshold resistor Rthr. As in FIG. 4A, the emitter resistor Rtg determines the gain of the thermal derating unit TDU.
No further figure is required to explain that in the event of changing the sequence of the elements in the voltage divider which defines the activation threshold temperature, the complementary bipolar transistor in accordance with FIG. 4A or FIG. 4B is used in each case. Of particular interest is the combination of a PNP transistor connected to V1 in conjunction with an NTC which is connected to the base of the transistor and the common ground line LED−.
As can be seen from FIG. 5, a first control circuit RK1 provides the nominal current value in the form of the voltage Vout. For adjusting the voltage Vout current information is evaluated in the form of the current ICL, which is determined as a function of the resistor Rsct. A second control circuit, designated RK2 and not shown in detail, is for controlling the actual current value Iout, wherein for this purpose the voltage Vmess which decreases over a measuring resistor Rmess is compared with the voltage Vout. A typical value of Iout is, for example, 500 mA, whereas the measuring resistor Rmess can be 2Ω, for instance. FIG. 5 shows a very simple variant of the circuit configuration of the interface for simpler power supply units PSU in which high precision is not required.
Due to the requirement for as few connection lines as possible and the concept of a common ground line LED−, the problem arises of a voltage decrease on this common ground line caused by the operating current of the at least one light source module. The embodiment uses a very simple circuit based on a single operational amplifier without any equalization of a voltage offset on the common ground line due to the light source module current. Said single operational amplifier OpAmp of the power supply unit interface is at its inverting input connected to the communications line CL and at its non-inverting input connected to the already known voltage Vk, which due to its direct relationship to the common ground line LED− forms the reference for the interface circuit of the power supply unit PSU. The amplifier output is connected via the current measurement resistor Rfb to the inverting input through which the obligatory negative feedback of the operational amplifier is achieved. Its property of wanting to match the potential of both its inputs produces the reference voltage Vk on the communications line CL. As both its inputs exhibit very high ohmic values, practically no currents flow there. Therefore the current through Rfb is identical to the current ICL coming out of the connection of the power supply unit for the communications line CL and can only find its way back to the power supply unit via Rsct1 or Rsct and via the common ground line LED−. This current is measured by way of Rfb and generates an internal measuring signal Vout the value of which corresponds to the voltage Vk increased by the measuring current ICL multiplied with the current measurement resistor Rfb. As Vk is known, with ICL the value of Rsct1 and Rsct is also known. This is therefore a (simple, proportionally inverting) control circuit RK1 as the interface reference voltage Vk is not generated by the voltage source of the same name directly, but by the output of the control amplifier OpAmp. The output voltage Vout representing the nominal value of the total current requirement thus results almost “along the way”. This measuring signal Vout serves as an input signal for the second control circuit RK2 which compensates dynamics and faults of the power component CG, and sets and controls the LED operating current Iout to be supplied by the power component CG to the output of the power supply unit. The output of the power supply unit is connected to LED+ and the common ground line LED−, i.e. to the supply lines of the at least one light source module LEM.
The measuring error due to the voltage drop on LED− caused by the operating current of the at least one light source module, can be reduced by selecting an adequate value for Vk to a value suitable for the application in question. In an example of a variant, the maximum measuring error on the ground line is set to 50 mV. This is the equivalent of a current of 1 A on a 50 mΩ connection. Setting the measuring error to this produces 5 V as the smallest value for the voltage Vk so that Vout has an error caused by the voltage drop of less than 1%.
In order to achieve better accuracy, other methods of compensating against the voltage drop on the common ground line may be used. One method is to switch off the operating current for the at least one light source module before measuring Rset. This measuring can be done by a delayed release of the operating current when switching on the overall system.
It should be noted that when switching off the chain of light source modules, by removing the power on the supply line LED+, the active current level on the communications line CL is not influenced by the temperature signal. This is not a disadvantage, because this information is not required if the light source modules are completely switched off, but it is a way to read the value of Rset not only with higher accuracy, but also without any deviation through possible overheating. The reading thus takes place without any deviation caused by the respective light source module temperature.
The pure temperature information is obtainable, however, by simply separating the reference voltage Vk from the non-inverting input of the operational amplifier OpAmp and through connecting this input to the common ground line. This causes the voltage on the communications line CL to become approximately zero and the current in CL is thus independent of the value of Rset. Consequently the current in CL is now only a function of the light source module temperature. In the case of multiple connections, i.e. several connected modules, the current is a function of the module with the highest temperature. This enables the power supply unit operating the light source modules to reduce the operating current to these modules right from the start and to determine the current operating temperature of the light source module, even if it is not overheated. For a high measuring accuracy of the temperature when the light source modules are in steady-state operation it is advantageous if Rset is known.
In the also not yet published DE 10 2012 224 349.6, date of application also 21.12.2012, the applicant has also solved a further problem which has not yet been described here. In broad terms this involves processing the offset voltages or impedance adaptations constantly occurring during the processing of current information by way of an emitted voltage in the interior of a power supply unit. Particularly problematic is the output current measuring voltage Umess evident in FIG. 5 through which the reference mass is divided into a “higher half” and a “lower half”. If, as is shown in FIG. 5, the lowest point of Vk is connected to the “higher half”, the first control circuit RK1 operates faultlessly for nominal value determination, but in order to control the LED operating current Iout the second control circuit RK2 either has to be able to process negative actual value signals or has to cope with two reference potentials. If, on the other hand, and contrary to FIG. 5, Vk is connected to the other side of Rmess, i.e. to the “lower half” of the reference mass, the reference potentials for both control circuits may be the same, but the nominal value formation is falsified by the measuring value Vmess of the LED output current. As for reasons related to the principle both the problems do not arise in the solutions proposed here, detailed citations from DE 10 2012 224 349.6 can be dispensed with here.
FIG. 6 shows a temperature-dependent characteristic curve field of the power supply unit. The set of curves shows the internal control voltage Vout of the power supply unit via the temperature of the at least one light source module. The individual curves are based on the current requirement of the currently connected at least one light source module. As can be clearly seen the thermal derating starts at a temperature of about 93° C., until at about 100° C. to 104° C., the supply of the operating current is switched off completely.
The function of the interface is explained below using a practical example. As can be seen in FIG. 6, an internal measuring signal Vout of 10 V results in an output current of 1 A. The interface should be designed in a way that a conductance of 1 mS for Rset results in an output current of 1 A. According to FIG. 6 the voltage source Vk is set to 5 V. This means that 5 V are applied to Rset (see FIG. 5). The operational amplifier operates in a manner to minimize the level difference at its two inputs, which is made possible by its negative feedback via Rfb. Thus, when Vk is 5 V, this means that 5 V are also applied at the inverting input of the operational amplifier. This results in 5 V at the respective current setting resistor Rset and to a current through the communications line CL of
            5      ⁢      V              1      ⁢                          ⁢      k      ⁢                          ⁢      Ω        =      5    ⁢                  ⁢          mA      .      These 5 mA flowing through the communications line CL also flow through current measurement resistor Rfb, as the input of the operational amplifier has a high impedance and therefore a negligible current consumption. As the voltage of the internal measuring signal Vout in accordance with FIG. 6 for the desired operating current should be 10 V, the voltage across the current measurement resistor Rfb must also be 5 V, resulting in a resistance value of 1 kΩ respectively 1 mS for Rfb. According to this example, a light source module with a current requirement of 2 A would have a current setting resistor Rset of 2 mS or 500Ω.
As already mentioned, the three-line interface has the disadvantage that the measuring signal is falsified by the voltage drop on the common ground line LED− caused by the operating current of the at least one light source module. After all, the measurement current passes through the common ground line LED− along with the LED operating current.
FIG. 7 shows the characteristic curve of the current measuring unit CMU, which is mainly dependent on the current measurement resistor Rfb. The characteristic curve shows the internal signal Vout of the output of the current measuring unit CMU against the normalized current measurement resistor
      Rfb    RsetMin    .Rsctmin is the minimum allowable value of the at least one current setting resistor, which leads to the maximum specified output current Ioutmax of the power supply unit PSU. Thus, with the shown value of 1, if Rfb=Rsctmin, the power supply unit delivers its maximum current at a given voltage of the at least one light source module, thus also its maximum power at its output. The internal measuring signal Vout belonging to the maximum power is 2*Vk, as described in the example for FIG. 6.